Antonio Guterres added that Syria had become a "disgraceful humanitarian calamity, with suffering and displacement unparalleled in recent history".

The recent wave of emigration from Syria may even mask the true number. British Red Cross disaster manager Pete Garratt said it was likely to be higher.

"We know there are people who will not have registered for support, for many reasons. They may be afraid of any form of authority or of registering their status," he said.

"In Jordan, the majority [70%] of refugees are living in urban areas away from the camps, presenting additional challenges for agencies in both finding the families who need support, and getting the aid to them."

The vast majority of Syria's refugees are hosted by countries in the neighbouring region (97%), which needs massive international support to help deal with the crisis.

An average of 5,000 Syrians are forced to flee into neighbouring countries every day. Ministers from Iraq, Jordan, Lebanon and Turkey are due to meet with UNHCR in Geneva on Wednesday in a bid to accelerate international support.

Over half of the 2 million refugees (52%) are children aged 17 years or under. Just days ago, the UNHCR announced that the number of Syrian child refugees had passed 1 million.

The UN special envoy Angelina Jolie said the world was "tragically disunited" on how to end the Syria conflict, adding: "The world risks being dangerously complacent about the Syrian humanitarian disaster.

"The tide of human suffering unleashed by the conflict has catastrophic implications. If the situation continues to deteriorate at this rate, the number of refugees will only grow, and some neighbouring countries could be brought to the point of collapse."

Screen grab from the UNHCR showing the number of people to have fled Syria since the conflict started (UNHCR)